Domains: Standardize store and module getters

[stephanethomas]

This pull request seeks to standardize the few access methods that weren't following the de facto naming convention in Flux stores (used in the domains management section) as well as in a module (used in the checkout). The logic didn't change and everything should work as before.

Testing instructions

Checkout

1. Navigate to the Domains Search page
2. Click on any Add button to add a domain to the shopping cart
3. Click on the No thanks, I don't need email or will use another provider. link on the next page
4. Select a country on the Domain Registration Details page that triggers the list of states (e.g. Spain)
5. Fill in and submit the form
6. Submit the form on the Secure Payment page and check that the purchase was successful

Domains Management

1. Navigate to the Domains page
2. Click on the new domain registration to open the corresponding Domain Settings page
3. Check that you can still perform any operation such as:
   • Making the domain primary
   • Adding an email forward
   • Adding a DNS record
   • Updating name servers

[gziolo]

Code changes look good and app works like before 👍